Twins, their age-matched siblings and parents are studied longitudinally to obtain an accurate estimate of facial, dental and somatic growth, including their interrelationships, with reference to adolescent growth rates and patterns, physique, physiologic and chronologic age scales. The contribution of genetic and environmental influences to individual differences in growth rate, size and timing of development will be determined together with an assessment of the short term predictability of facial growth trends. A total of 531 twin pairs have collaborated in this study since 1959. The main twin sample consists of 414 twin pairs, averaging nine years of follow-up, with 160 male pairs, 134 female pairs, and 120 male-female pairs. On the basis of blood typing analyses, 94 male pairs and 88 female pairs were classified as monozygotic twins. More than 400 siblings of the twins have also participated in this longitudinal study.